An uncuffed murder suspect duped a Brooklyn cop into opening his holding-cell door so he could use the toilet — and instead barreled the officer over and ran right out the front door, authorities said.

Brandon Santana, 24, remained at large last night amid a massive manhunt — and as the devastated family of his alleged victim demanded answers about how he was allowed to slip out of the 78th Precinct station house.

“The precinct fell asleep on him,” said Yolanda Santiago, 48, whose 22-year-old son, Alexander, was savagely bludgeoned to death on Aug. 1, 2010. “Where were the other cops?”

Alexander’s girlfriend, Stephanie Mercado — the mother of his son, Alexander Jr. — said Santana’s escape is “like reliving the pain all over again.

“I’m an emotional wreck,” she said.

Santana is suspected of repeatedly bashing Alexander Santiago with a lead pipe during a gang assault on the man and three of his friends at 12th Street and Fifth Avenue in Park Slope.

One of the victims was stabbed more than 20 times, but survived.

The motive for the assault is unknown. One of the victims, not Santiago, was a confidential witness for law-enforcement, a source said.

Cops who were seeking Santana for the killing recently learned that Santana was “hiding out with his family” in Iowa, a source said.

“The NYPD learned that he was coming back to New York City, and they were waiting for him when he got here” earlier this week, the source said.

Detectives questioned Santana at the station house Wednesday night, and then left at the end of their shift at 1 a.m. yesterday, expecting to put him in a witness lineup later.

An hour later, after he had been placed in a first-floor holding cell, Santana asked the officer minding him if he could use the bathroom.

When the cop opened the cell door, Santana — who was not handcuffed — shoved the officer, knocking him to the ground, and ran straight out of the station house, law-enforcement sources said.

One cop behind the front desk jumped over it to chase him, but hurt himself in the process, sources said.

A lieutenant also went after Santana but couldn’t catch up, sources said.

“It’s like they gave us justice, then took it away,” said Anaisa Santiago, Alexander’s 15-year-old sister.

NYPD spokesman Paul Browne said, “The incident is under department investigation and the subject, Brandon Santana, is being actively pursued.”

The officer shoved to the ground was treated at Methodist Hospital for an elbow injury.

Anyone with information on Santana’s whereabouts is asked to call Crimestoppers at (800) 577-TIPS.